Future historians will look back at Britain in the age of Brexit and seek to explain why its people reduced their power and influence in the world in the belief that they were doing the exact opposite.

But historians will have to move quickly if they are to have a say because the most important consequences of Brexit are already with us. People do not see this because UK membership of the EU is wrongly discussed as an economic issue when it is primarily a political one.

This is a traditional mistake by the British who have been making it with varying degrees of intensity ever since the French politician Robert Schumann put forward his plan for the French-German Coal and Steel Community on 9 May 1950 – a pact that eventually turned into the EU. Enhancing the political power of European states, particularly France and Germany, was always the chief objective.

The misperception has meant that the outcome of the Brexit crisis is talked about with alarm or enthusiasm, depending on the views of the speaker, but generally on the supposition on all sides that this is something good or bad that lies in the future. There is ceaseless discussion about custom unions and single markets which masks the devastating loss of UK power and influence that has already occurred in three crucial areas.

There is greater political division in Britain than at any time since the 17th century and this is not going to go away. Even if parliament is finally forced to swallow Theresa May’s withdrawal agreement at least half the population is going to feel that they have been betrayed or, at best, are the victims of an act of self-destructive idiocy. Theresa May has systematically made these divisions deeper by pretending that there was national unanimity about the referendum decision.

Secondly, the UK as a state is more divided than it has been since the Scottish Act of Union in 1707. Scottish nationalists and Irish nationalists in Northern Ireland – half the population – are strengthened because Scottish independence and Irish unity can be presented as a jump into the future with the EU rather than staying put with a regressive and xenophobic England. Through resurrecting Irish partition as an issue, Britain will be permanently at odds with the Irish government, something which might not have mattered much in the past, but does now when Ireland is empowered by the rest of the EU. Angela Merkel was in Dublin this week telling Leo Varadkar that Germany would “stand by Ireland” and earlier the Irish taoiseach was seeing Emmanuel Macron in Paris. This used not to happen.

The third area in which the balance of power has swung against the UK over the last three years is that, as Britain becomes more divided, the EU becomes more unified. This was not inevitable: remember how there was talk in 2016 of the EU shedding more members and possibly even breaking up. Having seen what is happening to Britain, dissident members of the EU are these days keeping secessionist thoughts very much to themselves.

Evidence of the swing in the balance of power away from Britain has become more apparent over the course of the Brexit negotiations. Retreat on one side and advance on the other was the inevitable consequence of Brussels having much the strongest hand of cards. Nothing is more absurd, and a sign of a frightening detachment from reality, than the claim of prominent Eurosceptics that Britain would have got what it wanted in negotiations if only it had been firmer, something it failed to do because the British negotiators were weak-willed, incompetent or treacherously sabotaging their own side.

At one level, the explanation for the crisis gripping Britain is not a mystery: globalisation has produced political crises all over the world which differ in some respects but have certain common themes such as de-industrialisation, increased inequality, immigration, and the alienation of large parts of the population. There are obvious parallels between Trump supporters in Pennsylvania and Leave voters in the Labour strongholds in the Welsh Valleys. The same pressures were long visible in the Middle East where kleptocratic elites clustered around authoritarian rulers and their families, leaving the rest of the population to rot.

But this does not tell us why it is that the political crisis in the UK looks worse than elsewhere in Europe. Unsurprisingly, the historically minded have sought guidance from past precedent and, more particularly, from crises that revolved around the UK’s relations with continental Europe and, equally important, those that threatened the integrity of the UK itself.

Jacob Rees-Mogg caused some hilarity by plunging deep into the Middle Ages for an analogy, pillorying May’s withdrawal agreement as a sell-out by claiming that it was “the greatest vassalage since King John paid homage to Phillip II at Le Goulet in 1200”. Academic historians immediately pounced to call his history wrong, but there was nothing wrong with taking the longer view.

One could even go back a further 800 years and look at Roman Britain’s departure from the empire at the start of the fifth century. The circumstances are murky and ill-recorded, but there is evidence that the Romans did not pull out unilaterally but were encouraged to go by the local inhabitants. According to the sixth century historian Zosimus, quoted in The Anglo-Saxon World by Nicholas J Higham and Martin J Ryan, they were reduced “to such straits that they revolted from the empire, no longer submitted to Roman law, and reverted to their native customs. The Britons, therefore, armed themselves and ran many risks to ensure their own safety and free their own cities from attacking barbarians.”

This early example of Brexit did not end happily, though it would be interesting to imagine some Romano-British version of Liam Fox trotting off to sign trade deals with the Angles, Saxons and Jutes on behalf of “global Britannia”.

A constituency has always existed in Britain open to the idea that continental entanglements are a costly waste of money and are undertaken with the complicity of simple-minded or corrupt British politicians. Jonathan Swift wrote a devastating pamphlet, The Conduct of the Allies, making such charges three hundred years ago that would not look out of place in Leave campaign literature.

Britain has traditionally tried to avoid having a crisis in relations with continental Europe powers at the same time as a crisis threatening the unity of the UK. But Brexit was guaranteed to produce both simultaneously. Eurosceptics denigrate the EU as a leviathan and then seem shocked when it behaves like one. “Splendid isolation” was a dangerous idea for Britain even when it was at the height of its power at the end of the 19th century and looks like an even worse one today. Even so, Leavers are blithely confident that it does not much matter that Britain is now more isolated in Europe than at any time since the five-year period before Napoleon marched on Moscow in 1812.

The failure of historians to find a convincing parallel between events in Britain’s past and the Brexit crisis may have a simple explanation: never before has the nation embarked on a project likely to make it poorer, weaker and less able to control its own future.